The state-of-the-art electrophotographic process copying machines or printers are far inferior to lithography or silver salt system photography in image quality. In an attempt to improve the image quality of electrophotographic process copying machines or printers, various efforts have been made to improve toners and carriers constituting the developer, image-forming apparatus, etc.
For the part of toner, it has recently been necessary more and more to reduce the particle diameter of particulate toner in order to improve image quality such as resolution.
Various technical developments have been made. However, most of powder toners for development of electrostatic image commercially available at present have a volume-average particle diameter of from about 8 to 13 .mu.m. Powder toners having the smallest particle diameter have a volume-average particle diameter of about 7 .mu.m (as measured by Coulter Multisizer). Thus, the smallest allowable volume-average particle diameter of particulate toners extremely useful for the enhancement of image resolution is about 7 .mu.m at present. No particulate toners having far smaller particle diameters are commercially produced. Little or no developing machines using such a small particle size toner have been developed.
A powder toner is prepared by a dry process such as pulverization process or a wet process such as polymerization process and so-called phase inversion emulsification method as described in JP-A-5-66600 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-09-311502. It is said that the smallest allowable particle diameter of toners produced by a pulverization process using the present crushing machine on an industrial basis is about 6 to 7 .mu.m. Of course, small particle diameter toners having a particle diameter of about 5 .mu.m can be produced. However, these toners cannot hardly be considered practical because they add to cost and exhibit deteriorated triboelectricity or powder fluidity caused by the reduction of the particle diameter thereof.
The wet process such as polymerization process and emulsification process is said to be essentially free from difficulty for the reduction of the particle diameter of powder toners. However, the prior art wet process toner is mainly intended in the stage of development or production to replace the foregoing pulverization process toner having an ordinary volume-average particle diameter range (about 7 to 13 .mu.m). Electrostatic image developers comprising small particle diameter toners having a volume-average particle diameter of about 6 .mu.m or less have been so far little studied. No practical formulations have been known.